You may have to tap it a little in different directions with a Soft Blow Hammer. Once you get it off,if you can get a bite on the screw, try putting some heat to the stator were the screw is at before trying to remove it.

Got the cone off this morning, now how do I get further in to get the impeller off? There seems to be some kind of "shaft-stub" inside where the cone was and it spins with the impeller. How do I get in there?

It feels pretty smooth but the current impeller is dirty as hell like I said (I'll post pictures when I get home from work) it is just covered in mud which I think might be causing a little bit of the gritty feel.